Some wireless networks, such as Bluetooth, are wireless replacements for low data-rate wired connections, such as parallel and serial cables. Bluetooth networks operate in the 2.4 GHz Industrial, Scientific, and Medical (“ISM”) band as described in Bluetooth Specification Version 1.1 dated Feb. 22, 2001, herein incorporated by reference. The 2.4 GHz ISM band is an unlicensed band (meaning that anyone may use the spectrum of this band as long as they comply with the specified spectrum usage regulations.). This band has been set aside by the Federal Communications Commission (“FCC”) of the United States. Several other countries, including the majority of Europe, have also set aside this radio frequency (“RF”) spectrum as an unlicensed band. Some of the other technologies that occupy this unlicensed band include IEEE 802.11b wireless networks, cordless phones and microwave ovens.
Many wireless protocols, such as Bluetooth, are generally interference limited systems meaning that interference can limit throughput and packet transfer rates. The SINR (signal to interference and noise ratio) requirement of wireless systems is such that the signal needs to be strong enough so that it can be discerned over system interference and noise, which enables data i.e. a packet to be demodulated. Some exemplary sources of interference in 2.4 GHz band include 802.11b networks, cordless phones, and microwave ovens. Interference is the dominant source of degradation in Bluetooth systems. For example, interference from an 802.11b network degrades packet transmission, and degrades the ability to decode a packet. Even if there is no interference, Bluetooth may be limited by other factors, such as multipath problems. Multipath problems refers to signal reflections from walls, trees, buildings, and other physical objects that cause difficulty in signal reception and translation.
Presently, a Bluetooth network can include a single master and up to seven slaves. The master and the slave(s) are required to transmit on an even and an odd time slot, respectively. Time slots dedicated to the master are referred to as master-to-slave time slots, while the time slots dedicated to the slave are referred to as slave-to-master time slots. Each time slot spans 625 microseconds. Depending on the packet type selected by either the master or the slave, a Bluetooth packet can occupy 1, 3, or 5 time slots. The maximum data rate that a Bluetooth device can support is 1 Mbps. Over time, a Bluetooth signal actually occupies 79 MHz of the available 83.5 MHz in the ISM band.
Bluetooth networks use a frequency hopping mechanism, meaning that a Bluetooth transmission changes the transmission frequency after each packet. A frequency is assigned to each slot. These frequencies depend on the master address and clock. When a Bluetooth device transmits a packet that occupies more than one (1) time slot, it holds the same frequency for the entire packet length. After the transmission is complete, the Bluetooth device resumes with the frequency for the next slot. Accordingly, when another device is operating at a frequency that is also used by the Bluetooth network, mutual interference between Bluetooth and the other device operating in the 2.4 GHz ISM band can and does occur. From the perspective of a Bluetooth device, these other devices appear as static interferers. The net effect of this interference is an increase in the packet error rate and a corresponding decrease in the effective data transfer rate (throughput). In fact, in the presence of interference, the data transfer rate (throughput) may drop to only a fraction of the maximum.
Frequency-Hopping Spread Spectrum (“FHSS”) is a system that changes frequency (also called “hopping”) at a rate of 1600 Hz over seventy-nine (79) 1-MHz-wide channels (typically numbered channel zero, channel one, etc.). Adaptive frequency hopping (“AFH”) is a method for adaptively modifying the hopping sequence to intelligently hop around interference in order to minimize signal collisions and increase data throughput and reliability. Frequency hopping Bluetooth systems normally assign 1 MHz of bandwidth to each of the seventy-nine (79) frequency channels, switching 1,600 times per second. With AFH, a Bluetooth system first measures the ratio of packet loss in the pre-communication, and selects about fifteen (15) frequency channels with the lowest interference. This approach makes it possible to send and receive Bluetooth data without affecting the 22 MHz bandwidth used by IEEE 802.11b and vice versa. As there is no throughput loss for either, and IEEE 802.11b does not have to be modified, AFH is highly practical.
The current approach for adaptive frequency hopping within the Bluetooth Coexistence Working Group is to hop over a reduced set of hopping frequencies that are deemed to be free of interference; in other words, to hop over only the good channels. A simplified block diagram for this adaptive frequency hopping mechanism 100 is shown in FIG. 1. Master clock 103 and master address 105 are fed into Legacy Hop Kernel 110 which generates hopping frequency fk 115 (fk can be defined as the next hop-frequency such that fk∈[0, , 78]). Hopping frequency fk 115 is then used as an input to Frequency Re-Mapper 120 which produces next adapted hop-frequency fadp 125 (fadp∈SG, where SG represents a set of good channels). In this scheme, the good channels essentially pass through Frequency Re-Mapper 120 unchanged, while the bad channels are uniformly re-mapped onto the set of good channels. If there are enough (NG) good channels in the band to satisfy the regulatory constraints (Nmin), i.e., NG≧Nmin, then the proposed mechanism works quite well. However, if there are not enough good channels in the band (e.g., when several 802.11b access points are located in close proximity), then bad channels (from SBK, where SBK represents a set of bad channels to be kept in the adapted hopping sequence) have to be inserted into the adaptive hopping sequence in order to reach the minimum number (Nmin) of hop-frequencies. These bad channels can have a serious impact on the throughput and packet loss rate. As more technologies enter the 2.4 GHz ISM band, the number of available clear good channels will decrease.
For a synchronous connection-oriented (“SCO”) link, a single bad channel can result in the loss of an entire voice packet. Since there is no automatic retransmission request (“ARQ”) protocol for SCO, this voice data can never be recovered. For an asynchronous connection-less link (“ACL”), the transitions between good and bad channels result in wasted resources and/or retransmissions. Since bad channels occur randomly in current hopping sequence methods, the loss of voice packets for an SCO connection or wasted resources and/or retransmissions for an ACL connection cannot be avoided.
It is therefore desirable to provide an adaptive frequency hopping solution that has low computational complexity while still mitigating the interference effects of bad channels. The present invention can provide this by using a structured adapted hopping sequences that reduces the occurrences of transitions between good and bad channels and/or assign good channels to slots where data is to be transmitted and bad channels to idle slots. The adaptive frequency hopping mechanism of the present invention is flexible enough to hop over only the good channels when the number of good channels available meets or exceeds a minimum required number of good channels, and can also generate a structured adaptive hopping sequence to reduce the effects of interference when bad channels must be used. The present invention can be designed to comply with current FCC regulations, yet remain adaptable to changing rules.